Well, congrats on posting your first ever fanfiction and for navigating through it all and figuring it out on your own. Good job.

The beginning of your story was very fairytale-like with mystery and intrigue with the royal couple and their baby. You did really well in the description. However, I really don't know what happened. Did the foal give up it's life and then the Queen turn into the foal? I might've missed something? Sorry.

I like the similarities to platform 9&3/4. I really liked the mirror as a door to the magical world. That was a really cool idea.

It is too bad that she doesn't feel these three could be friends, especially since she blushed with the guy... possible romance? Hmm? It is sad that she feels that it won't last.

For English being a second language, you did really, really well. One thing that stands out as far as language is concerned is that generally, I wouldn't say that someone is "longer." People are either taller or shorter. And also, I have no idea how tall 175 centimeters is. Haha. :P Since people around the globe have different ways of measuring things, you might just want to say that he was so tall that the top of her head barely touched his shoulder... or something else that people could relate to. Just a friendly suggestion, of course. You can certainly keep it if you want. :)

Well, good luck to you with all of your writings. You are off to a great start.